The present invention relates to photoelectric current integrating circuits for integrating photoelectric currents detected at photodiode, and more particularly relates to photoelectric current integrating circuit where the photoelectric currents detected at photodiode are integrated at low noise level, such as one for use in a photometric apparatus having the photodiode which is applicable for example to camera.
The construction shown in FIG. 1 as disclosed in Japanese Patent Application Laid-Open 2003-315149 is well known as an integrating circuit for converting photoelectric current detected at photodiode into voltage signals. The photoelectric current integrating circuit shown in FIG. 1 is constructed such that: cathode terminal and anode terminal of photodiode 501 are connected respectively to non-inverting input terminal and inverting input terminal of an operational amplifier 502; a constant voltage supply 503 having voltage value Er is connected to the non-inverting input terminal of the operational amplifier 502; an integrating capacitor 504 for accumulating photoelectric current and a switch 505 for resetting the integrating capacitor 504 are connected between the inverting input terminal and an output terminal of the operational amplifier 502; and an output terminal 506 of the photoelectric current integrating circuit is connected to the output terminal of the operational amplifier 502.
An operation of the photoelectric current integrating circuit having such construction will now be described. Before detecting photoelectric current, the switch 505 is closed so that in this condition the voltage Er of the constant voltage supply 503 is outputted to the output terminal of the operational amplifier 502. The switch 505 is then disconnected to start detection of the photoelectric current by the photodiode 501. Supposing the photoelectric current detected by photodiode 501 after passage of time t subsequent to the disconnection of the switch 505 as Ip, and value of the integrating capacitor 504 as Cint, output voltage Vout occurring at the output terminal 506 is represented as in the following equation (1), and a voltage signal is obtained.Vout=Er−Ipt/Cint  (1)
In the prior-art photoelectric current integrating circuit as disclosed in the above publication, error components not shown in (1) do occur. A factor occurring such error components is a noise caused mainly by an amplifier consisting of a parasitic capacitance Cst consisting of junction capacitance connected in parallel with the photodiode 501, an integrating capacitance Cint, and the operational amplifier 502. It should be noted that the noise occurring from the operational amplifier can usually be represented by an input-referred noise, and may be considered as equivalently occurring at an input terminal section of the operational amplifier 502. A prior-art photoelectric current integrating circuit with considering a parasitic capacitance 507 of photodiode 501 as Cst and input-referred noise 508 of the operational amplifier as En is shown in FIG. 2. At this time, supposing the error (hereinafter referred to as noise voltage) occurring at the output terminal 506 as Enout, the output voltage Vout is represented by the following equation (2).Vout=Er−Ipt/Cint+Enout  (2)
The noise voltage Enout in FIG. 2 will now be noticed to describe its operation. Supposing the change amounts when the analog switch 505 is disconnected of the electric charges accumulated at the parasitic capacitance Cst and at the integrating capacitance Cint due to the input-referred noise 508 respectively as ΔQ1, ΔQ2, and potentials of the inverting input terminal and non-inverting input terminal of the operational amplifier 502 respectively as Vin− and Vin+, the following equations are obtained.ΔQ1=Cst(Vin−+En−Vin+)  (3)ΔQ2=Cint(Vin−+En−Enout)  (4)ΔQ1+ΔQ2=0  (5)Enout=A(ω)(Vin+−Vin−)  (6)where A(ω) is an open-loop-gain of the operational amplifier 502, and is represented by the following equation (7).A(ω)=Ao/(1+jω/ωc)  (7)where ω is angular frequency, ωc is cut-off frequency, and Ao is open-loop gain under DC.
Further, since voltage Er of the constant voltage supply 503 is constant, the change amount ΔEr of Er when the analog switch 505 is disconnected is represented by the following equation (8).ΔEr=Vin+=0  (8)From equations (2) to (8), the following equation (9) is obtained.Enout =[(Cst+Cint)En+CintΔEr]/[(Cst+Cint)/A(ω)+Cint]  (9)
Here, if the operational amplifier is in its ideal condition, i.e. A(ω)=∞, equation (9) becomes the following equation (10), and the noise En of the operational amplifier is outputted as multiplied by (1+Cst/Cint).Enout =(1+Cst/Cint)En  (10)